Mesomorphism dependence on central bridges and the efficiency order of the tail group

Article abstract of DOI:10.1080/15421406.2016.1146937

A novel homologous series: RO-C₆H₄?CH?CH?COO?C₆H₄?CH?N-C₆H₃ (CH₃)₂ of ortho-meta Schiff's bases has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on mesomorphic properties and to provide a source of novel liquid crystalline (LC) materials. The series consists of 13 homologues (C₁ to C₈, C₁₀, C₁₂, C₁₄, C₁₆ & C₁₈). C₁ and C₃ homologues are nonmesomorphic, C₂ and C₄monotropicnematic, C₆ and C₇monotropicsmectic in addition to enantiotropicnematic, C₅ is only enantiotropicnematic, C₈, C₁₀, C₁₂ are enantiotropically smectogenic plus nematogenic and C₁₄, C₁₆, C₁₈ homologues are enantiotropically smectic only. Textures and transition temperatures were determined using an optical polarizing microscope equipped with a heating stage. Analytical and spectral data confirm the molecular structures. Average smectic and nematic thermal stabilities are 100.16°C and 110.83°C, respectively. Mesomorphic properties of present novel series are compared with structurally similar homologous series.

Full text of DOI:10.1080/15421406.2016.1146937

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Mesomorphism dependence on central bridges
and the efficiency order of the tail group
G. N. Bhola & U. C. Bhoya
To cite this article: G. N. Bhola & U. C. Bhoya (2016) Mesomorphism dependence on central
bridges and the efficiency order of the tail group, Molecular Crystals and Liquid Crystals, 630:1,
154-161, DOI: 10.1080/15421406.2016.1146937
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1146937
Published online: 01 Jul 2016.
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Date: 01 July 2016, At: 18:59

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Mesomorphism dependence on central bridges and the
efficiency order of the tail group
G. N. Bhola and U. C. Bhoya
Department of Chemistry, Saurashtra University, Rajkot, Gujarat, India
ABSTRACT
KEYWORDS
Schiff’s base; Mesomorphic;
Mesogen; Liquid Crystal;
Smectic; Nematic
–
=
–
–
–
=
A novel homologous series: RO-C6H4 CH CH COO C6H4 CH N-C6H3
(CH3)2 of ortho-meta Schiff’s bases has been synthesized and studied
with a view to understanding and establishing the effects of molecular
structure on mesomorphic properties and to provide a source of novel
liquid crystalline (LC) materials. The series consists of 13 homologues
(C1 to C8, C10, C12, C14, C16 & C18). C1 and C3 homologues are nonmeso-
morphic, C2 and C4monotropicnematic, C6 and C7monotropicsmectic in
addition to enantiotropicnematic, C5 is only enantiotropicnematic, C8,
C10, C12 are enantiotropically smectogenic plus nematogenic and C14,
C16, C18 homologues are enantiotropically smectic only. Textures and
transition temperatures were determined using an optical polarizing
microscope equipped with a heating stage. Analytical and spectral data
confirm the molecular structures. Average smectic and nematic thermal
stabilities are 100.16°C and 110.83°C, respectively. Mesomorphic proper-
tiesofpresentnovelseriesarecomparedwithstructurallysimilarhomol-
ogous series.
Introduction
Liquid crystal (LC) materials [1] are useful for many researchers in various fields of applica-
tions [2–6] either thermotropically or/and lyotropically. The present investigation is planned
with a view to generating novel homologous thermotropic liquid crystal materials of Schiff’s
bases with laterally substituted 2,5-dimethyl tail group in the absence of para substitution,
in order to study and evaluate mesomorphic behavior as a function of changing molecular
structure [7–10] as a consequence of changing molecular rigidity and flexibility [11–15]. The
mesogenic behavior of the novel thermotropic LCs are compared with other structurally sim-
ilar homologous series and the group efficiency order is derived on the bases of thermal sta-
bility, early commencement of mesophase and mesophase ranges. Several homologous series
and other LC materials [16–20] have been reported to date, but the proposed novel series
–
=
–
–
–
=
will consist of three phenyl rings and two central bridges, CH CH COO and CH N-
with flexible n-alkoxy (-OR) and laterally substituted 2,5-dimethyl group at the first and last
phenyl rings respectively. Mesogenic properties or behaviors of present novel series will be
interpreted on the bases of molecular rigidity and flexibility.
CONTACT U. C. Bhoya
drucbhoya@gmail.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
155
Scheme . Synthetic route to the novel series.
Experimental
Synthesis
4-n-Alkoxybenzaldehydes were synthesized by refluxing 4-hydroxybenzaldehyde (1 equiv.)
with corresponding n-alkyl bromides (1 equiv.) in the presence of potassium carbonate
(1 equiv.) and acetone as a solvent [21]. The resulting 4-n-alkoxybenzaldehydes were reacted
with malonic acid (1.2 equiv.) in the presence of 1–2 drops piperidine as catalyst and pyridine
as solvent to yield corresponding trans-4-n-alkoxy cinnamic acids (A) [22]. [{(2,5-dimethyl
phenyl) imino} methyl] phenol (B) was prepared by an established method [23] M.P.177–
179°C, Yield—76.5%. Coupling of compound A and B is done by Steglich esterification to
yield 4-(4ꢀ-n-Alkoxycinnamoyloxy) benzal 2ꢀꢀ,5ꢀꢀ-dimethyl aniline [24].
The synthetic route to the novel homologous series of Schiff’s base cinnamoyl ester deriva-
tives is under mentioned in scheme-1.
Characterization
Some of members of a novel series as the representative member of a series were charac-
1
terized by elemental analysis (Table 1), Infrared spectroscopy, HNMR spectra and mass

156
G. N. BHOLA AND U. C. BHOYA
Table . Elemental analysis for ()Propyloxy()Octyloxy()Dodecyloxy derivatives.
Elements %Found
Elements %Calculated
H
Sr.No.
Molecular formula
C
H
O
C
O



CHNO
CHNO
HNO
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
C
spectroscopy. Microanalysis was performed on EuroEA Elemental Analyzer. IR spectra
were recorded on Shimadzu FTIR Model-IRAffinity-1S (MIRacle 10), 1HNMR spectra were
recorded on Bruker spectrometer using CDCl3 as a solvent and mass spectra were recorded
on Shimadzu GC-MS Model No.QP-2010. The liquid crystal behavior and the type of textures
were determined by miscibility method on microscopic observations.
Analytical data
Spectral data
HNMR in ppm for ethyloxy derivative
–
1.17–1.21 (d,3H, CH3 of –OC2H5 group), 3.99–4.04 (m,2H,CH3-CH2-O), 7.46–7.48 (d,1H,
–
–
–
=
–
–
-CH = CH COO ), 6.41–6.45 (d,1H, CH CH COO ), 8.27 (s,1H, -CH = N-), 2.23–2.27
–
(s, 6H, two CH3 groups attached to benzene), 6.84–6.88 & 7.87–7.90 (4H, phenyl ring con-
taining ethoxy chain), 7.46–7.48 & 7.76–7.80 (4H, middle phenyl ring), 7.02–7.80 (3H, phenyl
ring containing two methyl ring). The NMR data are reliable with the molecular structure.
HNMR in ppm for pentyloxy derivative
–
–
– –
0.85–0.88 (t,3H, CH3 of –OC5H11 group), 1.29–1.32 (m,2H,CH3-CH2 (CH2)3 O )
–
–
–
– – – –
–
–
1.37–1.39 (p,2H, CH2 CH2 CH2 O ), 1.70–1.75 (p,2H, CH2 CH2 O ), 3.91–3.94
–
– –
–
–
=
–
–
–
–
=
–
–
(t,2H, CH2 O ), 7.45–7.47 (d,1H, CH CH COO ), 6.40–6.44 (d,1H, CH CH COO ),
= –
8.27 (s,1H, CH N ), 2.23–2.27 (s, 6H, two CH3 groups attached to benzene), 7.02–7.04
& 7.75–7.78 (4H, phenyl ring containing ethoxy chain), 7.45–7.47 & 7.75–7.79 (4H, middle
phenyl ring), 6.88–7.89 (3H, phenyl ring containing two methyl ring). The NMR data are
reliable with the molecular structure.
IR in cm- for butyloxy derivative
–
–
–
3020 (C H str. of alkene disubstituted), 2924 & 2866 (C H str. of ( CH2-)n group of –OC4H9),
=
=
1726 (C O str. of carbonyl carbon of ester group), 1629 (C C str. of alkene), 1598 & 1508
=
–
–
(C C str. of aromatic ring), 1309 & 1265 (C H bending of alkene), 1203 (C O str. of ether
–
–
linkage), 1132 (C O str. of ester group), 999,970 & 931 (C H bending of alkene). The IR data
are consistent with the molecular structure.
IR in cm- for octyloxy derivative
–
–
–
3037 (C H str. of alkene disubstituted), 2920 & 2850 (C H str. of ( CH2-)n group of –
=
=
OC8H17), 1720 (C O str. of carbonyl carbon of ester group), 1629 (C C str. of alkene),
1600,1571 &1506 (C C str. of aromatic ring), 1394 & 1319 (C H bending of alkene), 1246
(C O str. of ether linkage), 1118 (C O str. of ester group), 991, 968 & 923 (C H bending of
alkene). The IR data are consistent with the molecular structure.
=
–
–
–
–

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
157
Table . Transition temperatures in °C.
Compound No.
n-alkyl chain CnHn+ (n)
Sm
N
Isotropic


























—
—
—
—
—
(.)
(.)
.
.
.
.
.
.
—
(.)
—
.
.
.
.
.
.
.
.
.
.
.
.
.
(.)
.
.
.
.
.
.
–
—
—
Sm = Smectic, N = Nematic, () indicate monotropy
Mass spectra of methyloxy derivative
m/z (rel.int%): 385 (M)+,224,180,161,133,103,77
Mass spectra of heptyloxy derivative
m/z (rel.int%): 469(M)+,343,260,245,224,181,147,119,91,65
Result and discussion
Novel Schiff’s base cinnamoyl ester derived from trans n-alkoxycinnamic acid and [{(2,5-
dimethyl phenyl) imino} methyl] phenol is either monotropically or enantiotropically
nematogenic or/and smectogenic in character, except C1 and C3 homologue which fails to
exhibit mesomorphism. Transition temperatures as determined by polarizing microscopy
(Table 2) are plotted against the number of carbon atoms present in n-alkyl chain of left
n-alkoxy end group (Fig. 1), a phase diagram is obtained which consisted of Cr-I/M, Sm-
N/I, N-I or vice versa transition curves, showing phase behaviors of series by linking like
or related points. N-I and Sm-N/I transition curves exhibited odd-even effect. The Sm-N/I
transition curve for odd members occupy lower positions as compared to even members of
a series clearly whereas reverse is the case for N-I transition curve. N-I transition curve is
extrapolated [25–28] to nonmesomorphic C3 homologue from C5 homologue to determine
and predict its monotropicnematic transition temperature (122°C) which is actually not real-
izable. Transition curves for odd-even effect observed for smectic and nematic merges into
each other at C11 and C6 homologue respectively. Then, both curves propagated as a sin-
gle curve for higher homologue of longer n-alkyl chain from and beyond merging into each
other. Cr-M/I or vice versa transition curves follows a zigzag path of rising and falling with
overall descending tendency. Sm-N/I transition curve initially raises, passes through max-
ima at the C16 homologue and then descended in usual established manner. Similarly N-I
transition curve overall descended as series is ascended. Thus, all the transition curves of
a phase diagram behaved in normal manner. The mesogenic properties undergo variations
from homologue to homologue in the same novel series depending upon member of carbon
atoms present in n-alkyl chain (-OR) of left terminal end group.
–
=
Increasing length of a molecule by HO-C6H4 CH N-C6H3(CH3)2, increases molecu-
lar length to breath ratio, rigidity, flexibility, permanent dipole moment across the long
molecular axis, molecular polarity and polarizability, intermolecular dispersion and cohesion
forces including closeness etc., which causes favorable and suitable magnitudes of anisotropic

158
G. N. BHOLA AND U. C. BHOYA
Figure . Phase Behavior of Series.
forces of cohesion and closeness, which includes smectic or/and nematicmesophase either
monotropic or enantiotropic condition to stabilize suitable mesophase or mesophases within
definite ranges of temperature for homologues other than C1 and C3. The molecules of C1 and
C2 are, because of their low magnitudes of dispersion forces and low value of dipole-dipole
interactions are unable to resist exposed thermal vibrations. Hence, their (C1 and C3) crys-
tal lattices are abruptly breaking and transform into isotropic liquid without passing through
liquid crystalline state sharply at their malting point due to their high crystallizing tendency.
Rest of the homologues other than C1 and C3 are floating on the surface under exposed ther-
mal vibrations maintain molecular arrangement as required by formation of smectic or/and
nematic phase. The exhibition of odd-even effect is attributed to the number of odd and even
number of methylene unit or units present in n-alkyl chain of left n-alkoxy (-OR) group. The
disappearance of odd–even effect from and beyond merging of curves for odd and even num-
bered homologues is attributed to the unexpected status of longer n-alkyl chains by coiling
or bending or flexing or coupling with the major axis of a core structure of molecules, to
prolong as a single transition curve. The extrapolated N-I transition curve to C3, suggests
the monotropic transition temperature 122.0°C. However, it is not realizable due to its high
crystallizing tendency. The mesogenic behaviors of presently investigated novel series-1 are
compared with the structurally similar known series-X [29] as shown in Fig. 2.
Homologous series of present investigation-1 and a series chosen for comparison are iden-
tical with respect to (i) three phenyl rings and a left n-alkoxy terminal end group for the same
–
–
homologue and (ii) identical with respect to two functional groups CH3 or Cl; because the
–
–
–
–
mesogenic group efficiency of CH3 and Cl are almost equal or equivalent i.e. CH3= Cl.
The differing features of series-1 and X are the effect due to both central bridges which con-
tributes to the total molecular rigidity keeping almost identical molecular flexibility due to
equivalent group polarities of CH3 ࣈ Cl. Thus, the liquid crystal (LC) properties and
the degree of mesomorphism as depended upon thermodynamic quantity enthalpy (ꢀH) of
molecular structure which differs and depend upon changing magnitudes of intermolecular
–
–

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
159
Figure . Structurally similar series.
dispersion forces as a consequence of changing a part of molecular rigidity due to changing
–
=
–
–
–
–
–
= – – = –
the both central bridges; viz, CH CH COO or COO and CH N or N N in case
of series-1 and X respectively for the same homologue from series to series and from homo-
logue to homologue in the same series. Table 3 represents some changing features in terms of
thermometric LC properties for series-1 and X, under comparative study.
Table 3 indicates that,
r
Series-1 and X are smectogenic in addition to nematogenic.
Smectic thermal stability of both homologous series under comparison is nearly equiv-
alent with difference of about only six units.
Nematic thermal stability of present series-1 is lower than a series-X under comparison.
Smectogenic character commences from C6 member of a series-1 as well as series-X.
Nematogenicmesophase formation commences form C2 or C1 homologue for series-1
r
r
r
r
and X, respectively.
r
Melting and transition temperatures of present novel series-1 are relatively lower than a
series-X.
r
Total upper and lower mesophase length ranges for present novel series-1 are relatively
lower than a series-X.
r
Odd–even effect in the transition curves of a phase diagram is observed for both series.
The exhibition of smectogenic property identically from C6 homologue is attributed to the
similar magnitudes of combined effects of molecular rigidity and flexibility as occurred by
commencement of lamellar packing of molecules in crystal lattices which facilitate sliding
layered molecular arrangement in floating condition monotropically for series-1 and enan-
tiotropically for series-X under exposed thermal vibrations to include smectogenic character.
Moreover the extent of molecular noncoplanarity form and beyond C6 homologue of both
Table . Relative thermal stabilities in °C.
Series →

X
Smectic-Isotropic or
.(C–C)C
.(C–C)C
–
Smectic NematicCommencement
of smecticmesophase
Nematic-
.(C–C)C
.(C–C)C
IsotropicCommencement
of nematic phase
Total mesophase length
range (Sm+N)
. (C)to . (C)
.(C)to . (C)

160
G. N. BHOLA AND U. C. BHOYA
the series under comparative study which causes similar effect towards commencement of
smectic phase. Looking to the values of smectic thermal stability of series-1 and X which are
nearer to each other (100.16°C and 106.0°C) also very well support to equivalence of meso-
–
–
–
genic group efficiency of CH3 and Cl. Observed difference of about six units is attributed
–
to the positional difference of two CH3 (ortho & meta) group and two Cl (meta & para)
in series-1 and X, respectively. Two lateral CH3 group raises molecular polarizability while
one of Cl lateral group (meta) induces polarizability and other Cl group (meta) induces
polarizability and other Cl group (para) induces end to end attractions relatively more than
–
–
–
–
a series-1. Thus, intermolecular end to end attractions of series-X, exceeds the intermolecu-
lar lateral attractions through polarizability factor; which maintained and facilitated statically
parallel orientational order of molecules in floated condition more under the exposed thermal
vibrations. Therefore facilitation of nematic phase, though, commences from nearer (second
and first) homologues, but their thermal stabilities differ with wide range of about 56 units.
As regards to the upper and lower values of total mesophase length range and thermal sta-
bility for nematic which are higher for series-X than a present series-1; should be expected
higher for a series-1 as compared to series-X by apparent look of molecular structure because,
–
=
–
–
–
–
–
–
CH CH COO central group is relatively longer and stronger than identical COO cen-
–
=
tral bridge of series-X. However, contribution to molecular rigidity of CH CH COO plus
–
1 can reduce the end to end attractions as compared to COO plus N N central bridges
of series-X; which offers presence of para substituted Cl tail group and strangles end to end
= –
–
CH N central bridge due to absence of para substituted terminal tail group CH3 of series-
–
–
– = –
–
intermolecular attractions and cohesions to float on the surface with statistically parallel ori-
entational order for higher range of transition temperature, depending upon thermodynami-
cal quantity enthalpy change, based on respective molecular structure for the same homologue
from series to series and from homologue to homologue in the same series.
Conclusions
r
Novel Schiff’s base homologous series of LC family is predominantly nematogenic and
partly smectogenic whose mesophase length is relatively low and transition temperature
vary between 66.0°C and 139.0°C and of low thermal stability.
r
Mesogenic properties are dependent of the effects due to central bridges.
r
–
–
CH3 and Cl group are equipolar but 2,5 dimethyl and 3,4 dichloro groups are not
equipolar and equipolarizable due to their positional difference, affecting end to end
attractions and molecular flexibility.
Phenomena of mesomorphism are very sensitive and susceptible to molecular structure.
Smectogenic and nematogenic homologues of below 80°C may be useful for the devices
to be operated with the study of binary systems.
The present investigation has supported and raised the credibility of conclusion drawn
earlier.
r
r
r
Acknowledgments
Authors acknowledge thanks to the Department of Chemistry (DST-FIST Funded & UGC-SAP Spon-
sored), Saurashtra University, Rajkot, for research work. Authors are also thankful to Dr. A.V. Doshi,
Ex. Principal M.V.M. Science and Home Science College Rajkot, for his valuable co-operation during

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
161
present investigation as and when needed. Also thanks to the National Facility for Drug Discovery
through New Chemical Entities (NCE’s) for analysis of samples.
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